The present invention relates to antennas and, more specifically to meanderline loaded antennas having adjustable delay characteristics.
Efficient antennas typically require structures with minimum dimensions on the order of a quarter wavelength of their intended radiating frequency. Such dimensions allow an antenna to be easily excited and to be operated at or near its resonance frequency, limiting the energy dissipated in resistive losses and maximizing the transmitted energy. These conventional antennas tend to be large in size at their resonant wavelengths. Moreover, as the operating frequency decreases, antenna dimensions tend to increase proportionally.
To address shortcomings of traditional antenna design and functionality, the meanderline loaded antenna (MLA) was developed. A detailed description of MLA techniques is presented in U.S. Pat. No. 5,790,080. Wideband MLAs are further described in U.S. Pat. Nos. 6,323,814 and 6,373,440, while narrowband MLAs are described in U.S. Pat. No. 6,373,446. An MLA configured as a tunable patch antenna is described in U.S. Pat. No. 6,404,391. Each of these patents is herein incorporated by reference in its entirety.
Generally, an MLA (also known as a xe2x80x9cvariable impedance transmission linexe2x80x9d or VITL) is made up of a number of vertical sections and horizontal sections. The vertical and horizontal sections are separated by gaps. Meanderlines are connected between at least one of the vertical and horizontal sections at the corresponding gaps. A meanderline is designed to adjust the electrical (i.e., resonant) length of the antenna, and is made up of alternating high and low impedance sections. By switching lengths of the meanderline in or out of the circuit, time delay and phase adjustment can be accomplished.
U.S. Pat. No. 6,313,716, which is herein incorporated by reference in its entirety, describes a slow wave meanderline having sections of alternating impedance relative to a conductive plate. As explained in that patents the propagation constant of a meanderline is proportional to the square root of ZH/ZL, where ZH is the impedance of the high impedance sections of the meanderline, and ZL is the impedance of the low impedance sections of the meanderline. Thus, propagation delay through the meanderline can be adjusted by mechanically switching or switching out various sections of the meanderline.
Likewise, propagation delay through the meanderline can be adjusted by physically moving sections of the meanderline. Although such configurations allow for adjustment of the propagation constant of an MLA, they require an open/close switching scheme or some other electro-mechanical interface means to effect the adjustment.
What is needed, therefore, are improved techniques for effecting a change in propagation delay in an MLA.
One embodiment of the present invention provides a method for manufacturing a variable impedance transmission line (VITL). The method includes providing a conductive reference plane, and providing an activation layer between the conductive reference plane and one or more sections of a VITL conductor thereby defining a number of low impedance sections and high impedance sections. The activation layer has characteristics which change in response to electrical stimulus, which enables control over section impedance independent of antenna operating frequency.
Another embodiment of the present invention provides a VITL circuit. The circuit includes a conductive reference plane, and an activation layer that is operatively coupled between the conductive reference plane and one or more sections of a VITL conductor thereby defining a number of low impedance sections and high impedance sections. The activation layer is associated with an activation factor which changes in response to electrical stimulus, which enables control over section impedance independent of antenna operating frequency.
Another embodiment of the present invention provides a method for controlling propagation delay through a VITL. The method includes providing an activation layer operatively coupled between a conductive reference plane and a variable impedance transmission line conductor. The conductor has a number of high impedance sections and low impedance sections. The method further includes electronically stimulating the activation layer to change its characteristics so as to enable control over propagation delay through the variable impedance transmission line independent of antenna operating frequency.
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.